This invention relates generally to pigments, and more specifically relates to a process for producing high brightness pigments by beneficiation of naturally occurring dolomitic ores, such as dolomitic limestones.
The mineral dolomite, i.e. the double carbonate CaCO.sub.3.MgCO.sub.3 is present as a component of many natural ores. Most noteworthy of these ores for present purposes are the dolomitic marbles (or dolomitic limestones). In a typical instance an ore of this type will analyze at about 30% CaO and 20% MgO; but limestone ores are usually regarded as being of the dolomitic type when the MgCO.sub.3 content is in excess of about 4-5%.
Pigments derived from dolomitic ores find application in a wide variety of industrial and other environments. Such pigments, for example, can be used as fillers in the manufacture of rubber, of various plastics, and as extenders in paint formulations. In many of the aforementioned applications, particularly e.g. where the pigments are to be used for paper production, it is desirable that they be as bright as possible.
In addition it is desirable in many applications, to utilize a pigment of high purity, for reasons other than brightness alone. For example, in the preparation of certain insulating compositions, pigments derived from dolomitic ores may be used as an extender; and if metallic impurities such as pyrites are present, the electrical or electronic properties of the product can be adversely affected.
Interest has long centered upon possible use of naturally occurring dolomitic ores, such as dolomitic marble, particularly in that such natural minerals are extremely abundant in virtually all parts of the world -- and therefore represent a ready source of inexpensive raw material. In practice, however, it is found that very many of the dolomitic deposits are so highly contaminated with discolorants, that when comminuted in their natural state they are simply unacceptable as pigments. Thus, in typical instances deposits which are primarily of dolomitic limestone, may be contaminated with pyrites, hematite and with mica; all of which in varying degrees contribute to the discoloration of the otherwise relatively colorless material.
Various proposals have from time to time been advanced for processes directed at improving the brightness of the aforementioned naturally occurring dolomitic ores. By and large, however, the techniques of the prior art have not been particularly effective in producing high brightness pigments from natural dolomitic sources. Indeed in most instances where pigments are derived on a commercial scale from such natural sources, an ore is utilized which to begin with is of very high purity and relatively free from discoloring contaminants. Reference may be had in this connection, for example, to U.S. Pat. No. 3,592,674, which cites the use of pigments derived from a high purity natural dolomitic limestone -- which to begin with is essentially free of discoloring contaminants.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a process which enables the production of high quality bright dolomitic pigments, from naturally occurring dolomitic ores containing relatively high levels of discoloring contaminants.